Method and apparatus for delivering therapy in and association with an intravascular ultrasound device

ABSTRACT

An ultrasound transducer array ( 408 ) includes at least one transducer element ( 412 ) having a first ( 604 ) and second ( 606 ) portions separated by an acoustical discontinuity ( 520 ). The first portion ( 604 ) has the desired length to form a half-wave k31 resonance, while the second portion ( 606 ) has a resonant length for an undesired very low frequency out-of-band k31 resonance. The thickness of the transducer element ( 412 ) is designed for k33 half-resonance. Given the design, the transducer element ( 412 ) can operate and provide for both forward-looking ( 514 ) and side looking ( 512 ) elevation apertures. A method is also disclosed for using the disclosed ultrasound transducer ( 412 ) in ultrasound imaging.

This application is a Continuation of U.S. application Ser. No.10/174,412 filed Jun. 18, 2002 and now U.S. Pat. No. 6,780,157m which isa Continuation of U.S. application Ser. No. 09/501,106 filed Feb. 9,2000 and now U.S. Pat. No. 6,457,365.

TECHNICAL FIELD

This invention pertains generally to ultrasonic imaging and, moreparticularly, to an apparatus and method for ultrasonic imaging of smallcavities with catheter based devices.

BACKGROUND

In recent years, intravascular ultrasound (IVUS) imaging systems havebeen designed for use by interventional cardiologists in the diagnosisand treatment of cardiovascular and peripheral vascular disease. Suchsystems enhance the effectiveness of the diagnosis and treatment byproviding important diagnostic information that is not available fromconventional x-ray angiography. This information includes the location,amount and composition of arteriosclerotic plaque and enables physiciansto identify lesion characteristics, select an optimum course oftreatment, position therapeutic devices and promptly assess the resultsof treatment.

Such IVUS systems generally include an IVUS device having one or moreminiaturized transducers mounted on the tip of a catheter or guide wireto provide electronic signals to an external imaging system. Theexternal imaging system produces an image of the lumen of the artery orother cavity into which the catheter is inserted, the tissue of thevessel, and/or the tissue surrounding the vessel. Problems encounteredwith these systems include clearly visualizing the tissue around thecatheter, and identifying the precise location of the image with regardto known spatial references, such as angiographical references.

Some of the ultrasonic imaging catheters currently in use are “sideviewing” devices which produce B-mode images in a plane which isperpendicular to the longitudinal axis of the catheter and passesthrough the transducer. That plane can be referred to as the B-modelateral plane and is illustrated in FIG. 1. There are also “forwardviewing” devices that produce a C-mode image plane as illustrated inFIG. 2 which is perpendicular to the axis of the catheter and spaced infront of the transducer. Other forward viewing devices produce a B-modeimage in a plane that extends in a forward direction from the transducerand parallel to the axis of the catheter. That plane is referred to asthe B-mode forward plane and is illustrated in FIG. 3. The forwardviewing devices are particularly advantageous in that they allow thephysician to see what is in front of the catheter, and they also allowimaging in areas which cannot be crossed with the catheter.

One problem with imaging catheters heretofore provided is that a giventype of IVUS catheter can provide high quality images in only one of themode planes. There are no known devices, for example, which can provideboth B-mode forward imaging and C-mode imaging. As such there exists inthe art a need for an ultrasonic imaging transducer and method of theabove character which can overcome the limitations and disadvantages ofthe prior art as well as be able to provide images in multiple planes.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention, which are believed to be novel,are set forth with particularity in the appended claims. The invention,together with further objects and advantages thereof, may best beunderstood by reference to the following description, taken inconjunction with the accompanying drawings, in the several figures ofwhich like reference numerals identify like elements, and in which:

FIGS. 1–3 are isometric views showing different imaging planes generatedby an ultrasonic catheter tip.

FIG. 4 is a drawing of one embodiment an ultrasonic imaging catheterwith a guide wire and central distal lumen utilizing a ultrasonictransducer array assembly according to the present invention.

FIG. 5 is a side elevational view, partly broken away, showing oneembodiment of an ultrasonic imaging catheter incorporating theinvention.

FIG. 6 is an enlarged sectional view of one embodiment of an ultrasonictransducer element in accordance with the invention.

FIG. 7 is a side cross-sectional view of an alternate embodiment of atransducer element interconnection technique in accordance with theinvention.

FIG. 8 is a diagram of an ultrasonic transducer array assembly shownduring manufacturing in its flat state in accordance with the invention.

FIG. 9 shows a block diagram of an ultrasound system in accordance withthe present invention.

FIGS. 10–11 are diagrams showing the orientation of one C-mode imagevector and a description of the initialization of the element steppingaround the array necessary for the assembly of the vector in the image.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While the specification concludes with claims defining the features ofthe invention that are regarded as novel, it is believed that theinvention will be better understood from a consideration of thefollowing description in conjunction with the drawing figures, in whichlike reference numerals are carried forward.

Referring now to FIG. 4, there is shown a catheter 400 for intravascularuse. This catheter has an elongated flexible body 402 with an axiallyextending lumen 404 through which a guide wire 406, fluids, and/orvarious therapeutic devices or other instruments can be passed. Theinvention is not, however, limited to use with a catheter, and it can beutilized with any suitable catheter, guide wire, probe, etc. Anultrasonic imaging transducer assembly 408 is provided at the distal end410 of the catheter, with a connector 424 located at the proximal end ofthe catheter. This transducer 408 comprises a plurality of transducerelements 412 that are preferably arranged in a cylindrical arraycentered about the longitudinal axis 414 of the catheter fortransmitting and receiving ultrasonic energy.

The transducer elements 412 are mounted on the inner wall of acylindrical substrate 416 which, in the embodiment illustrated, consistsof a flexible circuit material that has been rolled into the form of atube. The end portions 420 of the transducer elements 412 are shown atthe distal portion of the transducer assembly. A transducer backingmaterial 422 with the proper acoustical properties surrounds thetransducer elements 412. An end cap 418, which isolates the ends of thetransducer elements, is attached to the transducer assembly.Alternatively, the end portions 420 of the transducer elements 412 canbe covered with nonconductive adhesive in order to insulate them fromexternal fluids (e.g., blood).

In FIG. 5 there is shown a cross-sectional view of the transducerassembly 408 cut in half and showing both sides. Integrated circuits 502for interfacing with the transducer elements are mounted on thesubstrate 416 and interconnections between the circuits 502 and thetransducer elements are made by electrically conductive traces 508 and522 on the surface of the substrate and buried within it. Conductivetrace 508 provides the “hot” conductor and interconnects the IC 502 to atransducer 412. Conductive trace 522 provides the ground conductorbetween the IC 502 and transducer 412. Interconnection vias 516, 518 and524 provide electrical connections between the conductive traces 508,510 and IC 502 and transducer element 512.

The transducer region has a central core that comprises a metal markertube 504 and plastic members 506 and 418, one acting as a lining 506 tothe lumen, and the other one is the end-cap piece 422 that acts both toprotect the end of the catheter and to act as a type of acousticcoupling enhancement for ultrasound transmission out the end of thearray. Between the central core of the catheter and the peripherallyarranged array elements there is an acoustic absorbing material 422. Thecore has a cylindrical body with an annular end wall at its distal endand an axially extending opening that is aligned with the lumen in thecatheter. A sleeve of metal 504 or other suitable radiopaque material isdisposed coaxially about the core for use in locating the tip of thecatheter within the body.

Each of the transducer elements 412 comprises an elongated body of PZTor other suitable piezoelectric material. The elements extendlongitudinally on the cylindrical substrate and parallel to the axis ofthe catheter. Each element has a rectangular cross-section, with agenerally flat surface at the distal end thereof The transducer elementsare piezoelectrically poled in one direction along their entire lengthas highlighted. A transversely extending notch 520 of generallytriangular cross-section is formed in each of the transducer elements.The notch opens through the inner surface of the transducer element andextends almost all the way through to the outer surface. Preferably, thenotch 520 has a vertical sidewall on the distal side and an inclinedsidewall on the proximal side. The vertical wall is perpendicular to thelongitudinal axis of the catheter, and the inclined wall is inclined atan angle on the order of 60° to the axis. The notch, which exists in allthe array transducer elements, can be filled with a stablenon-conductive material 526. An example of a material that can be usedto fill notch 520 is a non-conductive epoxy having low acousticimpedance. Although not the preferred material, conductive materialshaving low acoustic impedance may also be used to fill notch 520. If aconductive material is used as the notch filler, it could avoid havingto metalize the top portion to interconnect both portions of thetransducer elements as required if a nonconductive material is utilized.Conductive materials are not the preferred notch filler given that theyhave an affect on the E-fields generated by the transducer elements.

In the preferred embodiment, the transducer array provides for a forwardlooking elevation aperture for 10 mega Hertz (MHz) ultrasound transmitand receive 514, and a side looking elevation aperture 512 for 20 MHzultrasound transmit and receive. Other frequency combinations can beused depending on the particular design requirements. The inner andouter surfaces of the transducer elements are metallized to formelectrodes 528, 530. A secondary metalization is formed over theinsulated notch area 520 to create a continuous electrical connection ofthe electrode 530 between its proximal and distal ends. Outer electrodeserves as a ground electrode 530 and is connected by means of metal via518 to trace 522, which is buried in the substrate. Inner electrode 528extends along the walls of notch 520, wraps around the proximal end ofthe element and is connected directly to trace 508 on the surface of thesubstrate. In one embodiment, the transducer metallization consists of alayer of gold over a layer of chrome, with the chrome serving as anadhesion layer for the gold. Those skilled in the art will realize thatother metalization materials can be utilized.

The transducer array is manufactured by electrically and mechanicallybonding a poled, metallized block of the piezoelectric material 412 tothe flexible circuit substrate 416 with the substrate in its unrolled orflat condition as shown in FIG. 8. The transducer block exists, as apiezoelectrically poled state where the thickness-axis poling isgenerally uniform in distribution and in the same axis throughout theentire block of material. Notch 520 is then formed across the entirepiezoelectric block, e.g. by cutting it with a dicing saw. Each of theindividual notches 520 is filled with a material 526 such as plastic anda metallization 804 is applied to the top of the notch to form acontinuous transducer inner electrode with metallization 806. The blockis then cut lengthwise to form the individual elements that are isolatedfrom each other both electrically and mechanically, with kerfs 808formed between the elements. Cable wire attachment terminals 802 areprovided on the substrate which allow microcables that are electricallyconnected to an external ultrasound system to connect with thetransducer assembly in order to control the transducers.

The integrated circuits 502 are installed on the substrate 416, and thesubstrate is then rolled into its cylindrical shape, with the transducerelements on the inner side of the cylinder. The sleeve of radiopaquematerial is mounted on the core, the core is positioned within thecylinder, and the acoustic absorbing material is introduced into thevolume between the core and the transducer elements. In the event that aradiopaque marker is not required for a particular application, it canbe omitted.

The transducer elements 412 can be operated to preferentially transmitand receive ultrasonic energy in either a thickness extensional TE) mode(k₃₃ operation) or a length extensional (LE) mode (k₃₁ operation). Thefrequency of excitation for the TE mode is determined by the thicknessof the transducer elements in the radial direction, and the frequencyfor the LE mode is determined by the length of the body between distalend surface 614 and the vertical wall 610 of notch 520. The thickness TEmode is resonant at a frequency whose half wavelength in thepiezoelectric material is equal to the thickness of the element. And theLE mode is resonant at a frequency whose half wavelength in thepiezoelectric material is equal to the distance between the distal endand the notch. Each transducer element is capable of individuallyoperating to transmit and receive ultrasound energy in either mode, withthe selection of the desired mode (i.e. “side”, or “forward”) beingdependent upon; a) an electronically selected frequency band ofinterest, b) a transducer design that spatially isolates the echo beampatterns between the two modes, and c) image plane specific beamformingweights and delays for a particular desired image plane to reconstructusing synthetic aperture beamforming techniques, where echo timingincoherence between the “side” and “forward” beam patterns will helpmaintain modal isolation.

In FIG. 6 there is shown an illustration of a transducer element 412showing the E fields generated. In one presently preferred embodiment,the distance 604 between the notch 520 which forms an acousticaldiscontinuity and the distal end of each transducer element, alsoreferred to as the first portion of the transducer element, is madeequal to approximately twice the thickness 608 of the element, resultingin a resonant frequency for the TE mode that is approximately twice theresonant frequency for the LE mode. To assure good modal dispersionbetween the two modes, the TE frequency should be at least 1.5 times theLE frequency, and preferably at least twice. The steep wall 610 or sharpcut on the distal side of the notch provides an abrupt end to theacoustic transmission path in the piezoelectric material and facilitatesa half wave resonant condition at the LE frequency of operation.

The transducer element segment or second portion 606 which is betweenthe notch 520 and the proximal end 612 of the element will also be ableto resonate in an LE mode, but the design, through careful selection ofthis segment length, can be made to place this resonant frequency (andits harmonics) at a low (or high) enough frequency to create areasonable modal dispersion characteristic. The LE mode acousticcoupling for this segment to the “end” of the array will also be quitepoor, and aid in attenuating its undesired response. This segment thoughmay certainly participate in the TE mode excitation, to define the “sidelooking” aperture as the whole length of the piezoelectrically activeelement. Any element in the array can also be selectively used as areceiver of ultrasonic energy in either the TE or the LE mode ofoperation by the ultrasound system that is connected to the ultrasoundassembly.

In FIG. 7 there is shown an alternate interconnection technique used tointerconnect between the IC 502 and the individual transducer elements412. In this embodiment a solder or other conductive material such as ametal-epoxy 702 is used to connect ground trace 510 with the groundelectrode of the transducer element. An insulating separation layer 708between the ground and “hot” electrodes is located at the proximal endof the transducer element 412. In order to connect the “hot” conductivetrace or runner 508 to its corresponding electrode on the transducerelement, epoxy insulation 704 is used. The top of the epoxy insulationis then metallized 706 in order to electrically interconnect the traceor runner 508 to its corresponding electrode prior to dicing thetransducer block into discrete elements.

Multiple Modes of Imaging: Explanation of the Principals of Operation

A piezoelectric transducer, when properly excited, will perform atranslation of electrical energy to mechanical energy, and as well,mechanical to electrical. The effectiveness of these translationsdepends largely on the fundamental transduction efficiency of thetransducer assembly taken as a whole. The transducer is a threedimensional electro-mechanical device though, and as such is alwayscapable of some degree of electro-mechanical coupling in all possibleresonate modes, with one or several modes dominating. Generally animaging transducer design seeks to create a single dominate mode ofelectro-mechanical coupling, suppressing all other coupling modes as“spurious.” The common method used to accomplish a transducer designwith a single dominate mode of electro-mechanical coupling usually restsin the creation of a single, efficient mechanical coupling “port” to themedium outside of the transducer. The single port is created by mountingthe transducer such that the most efficient resonant mode of transduceroperation faces that mechanical coupling port, with all other modessuppressed by means of mechanical dispersion attained by transducerdimensional control and dampening materials.

In the design of the present invention, the transducer design utilizesthe fact that a transducer can be effective in two principalelectro-mechanical coupling modes, each mode using a different frequencyof operation, acoustic “port”, and electro-mechanical couplingefficiency. One port is the “side looking” port that is used in thecross-sectional view image as shown in FIG. 1. The other port is the“end”, or, “forward looking” port of the array.

The present invention allows the two electro-mechanical coupling modes(i.e. “side” 512 and “forward” 514) to be always active, without anymechanical switching necessary to choose one mode exclusive of theother. The design of this invention also assures that echoes of anyimage target in the “side looking” plane (see FIG. 1) do not interferewith the target reconstruction in the “forward looking” planes (seeFIGS. 2 and 3), and reciprocally, image targets from the “forwardlooking” do not interfere with the target reconstruction in the “sidelooking” planes. In accordance with the invention, the design methodslisted below are used to maintain sufficient isolation between the twomodes of operation.

A). Resonant and Spatial Isolation of the Two Modes

The “side looking” port is designed for approximately twice thefrequency of the “forward looking” port in accordance with the preferredembodiment. The transducer dimensional design is such that the “highfrequency and side looking” transducer port sensitivity to low frequencysignals, and as well the “low frequency and forward looking” transducerport to high frequency signals, is very low.

Additionally, the transmit and receive acoustic “beam” directions of thetwo modes 512 and 514 are at approximately right angles to each otherand this feature offers an additional isolation with respect to imagetarget identification. Also, as a means to further promote isolationbetween the two modes of operation, and as well optimize a sparse arrayecho collection method, the echo collection process in “forward” beamreconstruction uses an intentional physical separation of transmittingand receiving transducer elements of preferably 10 elements or more inthe circular array annulus. This physical separation aids in preventing“spurious” transmit echoes from the “high frequency side looking” portfrom contaminating the receiving element listening to “forward only”echoes at the its lower frequency of operation.

B). Electrical Frequency Band Isolation of the Two Modes

As stated previously, the two modes of operation are operated at centerfrequencies that differ by about a factor of two. This design featureallows for additional isolation between the two modes through the use ofband pass filters in the host system that is processing the echo signalsreceived from the catheter. Additionally, if one or both of the twomodes is operated in a low fractional bandwidth design (i.e. <30%), thebandpass filters will be even more effective in the maintenance of veryhigh modal isolation.

C). Beam Formation Isolation through Synthetic Aperture Reconstruction

Synthetic aperture beam reconstruction is used for all image modes. Thebeam formation process will preferentially focus only on image targetsthat are coherently imaged in a particular image plane. Thus, whileimage reconstruction is forming an image in, for example, the “sidelooking” plane, targets that may have contaminated the echoes from the“forward looking” planes will be generally incoherent and will besuppressed as a type of background noise. The reciprocal is also true:“side looking” echoes contaminants will be generally incoherent in“forward looking” imaging and will be suppressed through the process ofsynthetic aperture reconstruction.

A flexible digital image reconstruction system is required for thecreation of multiple image planes on demand. The preferred method ofassembling multiple image planes utilizes a synthetic aperturereconstruction approach. The “side looking” image shown in FIG. 1 can bereconstructed using sampled transducer element apertures as large as forexample 14 contiguous transducer elements in a 64 total transducerelement circular array. The transmit-receive echo collection foraperture reconstruction can be continuously shifted around the circulararray, sampling all transmit-receive cross-product terms to be used in aparticular aperture reconstruction. Within any 14-element aperture therecan be 105 independent transmit-receive echo cross products used toconstruct the image synthetically.

“Forward looking” images shown in FIGS. 2 and 3 can be reconstructedusing sampled apertures that consist of selected transducer elementsarranged on the annulus end of the circular array. For the 64 transducerelement example mentioned above, all elements may contribute to acomplete data set capture (this would consist of 64 by 32 independenttransmit-receive element cross-products) to form a “forward looking”image in either C-mode or B-mode. As an alternative to the complete dataset approach, a reduced number of independent transmit-receive elementcross-products are used to adequately formulate the image. Thetransmit-receive echo collection for aperture reconstruction can becontinuously shifted around the circular array, sampling alltransmit-receive element cross-products to be used in a particularaperture reconstruction.

Special signal processing may be advantageous, especially in the“forward looking” imaging modes that use a less efficient transducercoupling coefficient (k₃₁) and as well may suffer from additionaldiffraction loss not experienced in the “side looking” mode of syntheticaperture imaging. In forming a “forward looking” C-mode image plane asan example, a low noise bandwidth can be achieved by using a high numberof transmit pulses and a narrow bandpass echo filter in the processingsystem. Additionally, or as a preferred alternative, a matched filterimplementation from the use of correlation processing may be used toimprove the echo signal-to-noise ratio.

Standard Cross-Sectional B-Mode Operation

The advantage of this cross-sectional B-mode operation of the catheterimaging device is in its ability to see an image at great depth in theradial dimension from the catheter, and at high image resolution. Thisdepth of view can help aid the user of the catheter to position thedevice correctly prior to electronically switching to a “forwardviewing” mode of operation. Image targets moving quickly in a pathgenerally parallel to the long axis of the catheter can be detected anddisplayed as a colored region in this mode; this information can be usedto compare and confirm moving target information from the “forwardviewing” mode of operation of the catheter to enhance the usefulness ofthe imaging tool.

1. Transducer Operation

The transducer in this “primary” mode operates in the thicknessextensional (TE) resonance, utilizing the k₃₃ electro-mechanicalcoupling coefficient to describe the coupling efficiency. This“thickness resonance” refers to a quarter wave or half wave (dependingon the acoustic impedance of the transducer backing formulation)resonance in the transducer dimension that is in alignment with thepolarization direction of the transducer, and also the sensed or appliedelectric field. This TE mode utilizes a typically high frequencythickness resonance developed in the transducer short dimensionfollowing either electric field excitation to generate ultrasoundacoustic transmit echoes, or, in reception mode following acousticexcitation to generate an electric field in the transducer.

Array Stepping:

The TE mode is used for generating a cross-sectional B-mode image. Thiscross-section image cuts through the array elements in an orthogonalplane to the long axis of the transducer elements. Echo informationgathered from sequential transducer element sampling around the arrayallows for the synthetically derived apertures of various sizes aroundthe array. For the creation of any synthetically derived aperture, acontiguous group of transducer elements in the array are sequentiallyused in a way to fully sample all the echo-independent transmit-receiveelement pairs from the aperture. This sequencing of elements to fullysample an aperture usually involves the transmission of echo informationfrom one or more contiguous elements in the aperture and the receptionof echo information on the same or other elements, proceeding until allthe echo independent transmit-receive pairs are collected.

Notch Effect:

The small notch (520) forming an acoustical discontinuity in the middleof the array will have a minor, but insignificant effect on the TE modetransmission or reception beam pattern for that element. The small notchwill be a non-active region for the TE mode resonance and thereforecontribute to a “hole” in the very near field beam pattern for eachelement. The important beam characteristics however, such as the mainlobe effective beam width and amplitude, will not be substantiallyeffected, and except for a very minor rise in the transducer elevationside lobes, reasonable beam characteristics will be preserved as if theentire length of the transducer element was uniformly active.

Modal Dispersion:

The TE mode transducer operation will exist with other resonant modessimultaneously. The efficiency of electro-mechanical energy couplinghowever for each mode though depends on primarily these factors: a) thek coefficient that describes the energy efficiency of transduction for agiven resonance node, b) the acoustic coupling path to the desiredinsonification medium, and c) the echo transmission-reception signalbandwidth matching to the transducer resonance for that particular mode.Thus, for the creation of a “side looking” image, a transducer design iscreated to optimize the factors above for only the TE resonance, whilethe other resonant modes within a transducer are to be ignored throughthe design which suppresses the undesired resonances by minimizing theenergy coupling factors mentioned above.

Through this frequency dispersion of unwanted coupling, the desiredechoes transmitted and received from the “side looking” transducer portnecessary to create a B-mode image plane will be most efficientlycoupled through the TE resonance mode within any particular element.Therefore, the proposed transducer design which features a highefficiency TE mode coupling for desired echoes and frequency dispersionof the unwanted resonances and echoes, along with the other modalisolation reasons stated in an earlier section, constitutes a means forhigh quality TE echo energy transduction for only those desired in-planeechoes used in the creation of the B-mode cross-sectional image plane.

2. System Operation for the Standard Cross-Sectional B-Mode Imaging

The host ultrasound processing system shown in FIG. 9 controls theultrasound array 408 element selection and stepping process whereby asingle element 412 or multiple elements will transmit and the same orother elements will receive the return echo information. The elements inthe array that participate in a given aperture will be sampledsequentially so that all essential cross product transmit-receive termsneeded in the beam forming sum are obtained.

The host processing system or computer 914 and reconstruction controller918 will control the transmit pulse timing provided to widebandpulser/receiver 902, the use of any matched filter 910 via control line916 to perform echo pulse compression. The echo band pass filter (BPF)processing paths in the system are selected using control signal 906 toselect between either the 10 MHz 904 or 20 MHz 936 center frequency BPFpaths. The amplified and processed analog echo information is digitizedusing ADC 908 with enough bits to preserve the dynamic range of the echosignals, and passed to the beamformer processing section via signal 912.The beam former section under the control of reconstruction controller918 uses stored echo data from all the transmit-receive element pairsthat exist in an aperture of interest. As the element echo samplingcontinues sequentially around the circular array, all element groupapertures are “reconstructed” using well known synthetic aperturereconstruction techniques to form beamformed vectors of weighted andsummed echo data that radially emanate from the catheter surface usingbeamformer memory array 922, devices 924 and summation unit 926. Memorycontrol signal 920 controls switch bank 924 which selects which memoryarray to store the incoming data.

The vector echo data is processed through envelope detection of the echodata and rejection of the RF carrier using vector processor 928. Finallya process of coordinate conversion is done to map the radial vectorlines of echo data to raster scan data using scan converter 930 forvideo display using display 932.

This processing system, through the host control, may also accomplish ablood velocity detection by tracking the blood cells through theelevation length of the transducer beams. The tracking scheme involves amodification of the element echo sampling sequencing and the use of thebeamformer section of the host processing system. The blood velocityinformation may be displayed as a color on the video display; this bloodvelocity color information is superimposed on the image display to allowthe user to see simultaneous anatomical information and blood movementinformation.

Forward Looking Cross-Sectional C-Mode Operation

The advantage of this “forward looking” operation of the catheterimaging device is in its ability to see an image of objects in front ofthe catheter where possibly the catheter could not otherwise physicallytraverse. A “forward” C-mode plane produces a cross-sectional viewsimilar to the standard B-mode cross-sectional view, and so can offercomparable image interpretation for the user, and as well this forwardimage plane is made more useful because the user can see the presence ofimage targets at the center of the image, otherwise obscured in thestandard cross-sectional view by the catheter itself. This forward viewallows also the ideal acoustic beam positioning for the detection andcolor image display of Doppler echo signals from targets movinggenerally in parallel with the long axis of the catheter device.

1. Transducer Operation

The transducer in this “secondary” mode operates in the lengthextensional (LE) resonance, utilizing the k₃₁ electro-mechanicalcoupling coefficient to describe the coupling efficiency. In this modeof operation, the poling direction of the transducer element and thesensed or applied electric field in the transducer are in alignment, butthe acoustic resonance is at 90 degrees to the electric field and polingdirection. This “length resonance” refers fundamentally to a half waveresonance in the transducer element's length dimension that is at 90degrees with the polarization direction of the transducer. The LE modeof resonance, which is typically much lower in resonant frequency thanthe TE mode because the element length is normally much longer than thethickness dimension, always exists to some extent in a typicaltransducer array element, but is usually suppressed through a frequencydispersive design.

The preferred embodiment of the present invention utilizes an abruptphysical discontinuity (a notch 520) in the transducer element to allowa half wave LE resonance to manifest itself at a desired frequency, inthe case of the preferred embodiment, at about one half the frequency ofthe TE mode resonance. A unique feature of this invention is amechanically fixed transducer design that allows two resonant modes tooperate at reasonably high efficiencies, while the “selection” of adesired mode (i.e. “side”, or “forward”) is a function of a) anelectronically selected frequency band of interest, b) a transducerdesign that spatially isolates the echo beam patterns between the twomodes, and c) image plane specific beamforming weights and delays for aparticular desired image plane to reconstruct using synthetic aperturebeamforming techniques, where echo timing incoherence between the “side”and “forward” beam patterns will help maintain modal isolation.

As discussed earlier, a resonant mode in a transducer design can be madeefficient in electro-mechanical energy coupling if at least the threefundamental factors effecting coupling merit are optimized, namely a)the k coefficient (in this case it is the k₃₁ electro-mechanicalcoupling coefficient) that describes the energy efficiency oftransduction for a given resonance node, b) the acoustic coupling pathto the desired insonification medium, and c) the echotransmission-reception signal bandwidth matching to the transducerresonance for that particular mode. The invention allows for reasonableoptimization of these factors for the LE mode of resonance, although theLE mode coupling efficiency is lower than that of the TE mode coupling.The k₃₁ coupling factor, used in describing LE mode efficiency, istypically one half that of k₃₃, the coupling factor that describes theTE mode efficiency.

The abrupt acoustical discontinuity in the transducer element is createdat a step in the assembly of the array. Following the attachment of thetransducer material to the flex circuit to create a mechanical bond andelectrical connection between the transducer block and the flex circuit,while the transducer material is still in block form, a dicing saw cutcan be made the entire length of the transducer material block, creatingthe notch. The notch depth should be deep enough in the transducermaterial to create an abrupt discontinuity in the distal portion of thetransducer material to allow for a high efficiency LE mode half waveresonance to exist in this end of the transducer element. The saw cutshould not be so deep as to sever the ground electrode trace on thetransducer block side bonded to the flex circuit. The cut should ideallyhave a taper on the proximal side to allow for acoustically emittedenergy to be reflected up into the backing material area and becomeabsorbed.

It is not desirable that any acoustic coupling exist between the LEmodes of resonance in the distal and proximal transducer regionsseparated by the notch. The distal transducer region LE mode half waveresonance will exist at 10 MHz in PZT (Motorola 3203HD) for a length ofabout 170 microns between the distal end of the transducer element andthe notch. The proximal transducer region LE mode resonance will existat a frequency considered out of band (approximately 6 MHz) in the twoembodiments shown in FIGS. 5 and 7 so as to minimally interfere with thedesired operating frequencies (in this case 10 MHz LE mode resonance inthe distal region for “forward” acoustic propagation, and 20 MHz TE moderesonance in the entire active field length of the transducer).

The desired acoustic energy coupling port of the distal transducer LEresonant mode region is at the distal end of the catheter array. Toprotect the end of the array and potentially act as an acoustic matchinglayer, an end cap made of polyurethane could be used, or alternatively,a uniform coating of adhesive material would suffice. The beam patternproduced by this acoustic port must be broad enough to insonify a largearea that covers intended extent of the image plane to be formed. Tothis end, the beam pattern must typically be at least 60 degrees wide asa “cone shaped” beam measured in the plane to be formed at thehalf-maximum intensity angles for 2-way (transmitted and received)echoes. The preferred design of the array has 64 or more elements, and atransducer sawing pitch equal to pi times the catheter array diameterdivided by the number of elements in the array. For an effective arraydiameter of 1.13 mm and 64 elements, the pitch is 0.055 mm. Using twoconsecutive array elements as a “single” effective LE mode acoustic portcan provide an adequate, uniform beam pattern that produces the required60-degree full-width half maximum (“FWHM”) figure of merit. The apertureof this “single” forward looking port is then approximately 0.080 mm by0.085 mm (where 0.085 mm is twice the pitch dimension minus the kerfwidth of 0.025 mm).

The transducer design may also include a version where no notch isneeded in the transducer block. In this case, the driven electrode canexist all along one surface of the transducer element, and the ground orreference electrode can exist all along the opposite side of theelement. The long axis length of the transducer will resonate at a halfwavelength in LE mode, and the thickness dimension will allow theproduction of a TE mode resonance in that thickness dimension. In orderfor this design to operate though, the LE and TE mode resonantfrequencies will be quite different in order to maintain the proper TEmode elevation beam focus. As an example, in maintaining the length ofthe active region of the element for an adequately narrow 20 MHz TE modeelevation beam width at 3 mm radially distant from the catheter, theelement length should be approximately 0.5 mm long. The resulting halfwave resonance frequency in LE mode then will be about 3 MHz. Thisdesign can be used for dual-mode imaging, but will not offer thefocusing benefits that 10 MHz imaging can offer for the forward lookingimage planes. Other designs are possible, where the forward frequency ismaintained near 10 MHz, but the required frequency for the side-lookingmode will rise dramatically, and although this can be useful in itself,will complicate the design by requiring a concomitant increase in thenumber of elements and/or a reduction in the array element pitchdimension.

2. System Operation

The host processing system will control the array element selection andstepping process whereby one element, a two element pair, or othermultiple elements in combination, will transmit and the same or otherelements will receive the return echo information. The intended arrayoperational mode is the LE resonant mode to send and receive echoinformation in a forward direction from the end of the catheter array.As stated earlier, the LE mode echoes produced may be isolated from theTE mode echoes through primarily frequency band limitations (both bytransducer structural design and by electrical band selection filters),and through the beamforming reconstruction process itself as a kind ofecho selection filter.

To produce an image of the best possible in-plane resolution whileoperating in the forward-looking cross-sectional C-mode, the entirearray diameter will be used as the maximum aperture dimension. Thismeans that, in general, element echo sampling will take place at elementlocations throughout the whole array in preferably a sparse samplingmode of operation to gather the necessary minimum number ofcross-product echoes needed to create image resolution of high qualityeverywhere in the reconstructed plane.

By using transmit-receive echo contributions collected from elementsthroughout the whole catheter array, using either a “complete data set”(e.g. 64×32), or a sparse sampling (e.g. less than 64×32) of elements asshown in FIGS. 10 and 11, the FWHM main beam resolution will be close tothe 20 MHz resolution of the “side looking” cross-sectional image. Thisis due to the fact that although the “forward looking” echo frequency isabout one half as much as the “side looking” frequency, the usableaperture for the forward looking mode is about 1.6 times that of thelargest side looking aperture (i.e. the largest side looking aperture isabout 0.7 mm, and the forward aperture is about 1.15 mm). For a 10 MHzforward looking design, the FWHM main lobe resolution in an image planereconstructed at a depth of 3 mm will be approximately 0.39 mm, and 0.65mm resolution at 5 mm distance.

Due to the limitation of beam diffraction available in the design using10 MHz as the echo frequency for “forward looking”, the C-mode imagediameter that can be reconstructed and displayed with a high level ofresolution from echo contributions throughout the whole array will berelated to the distance between the reconstructed C-mode image plane andthe distal end of the catheter. At 3 mm from the end of the catheter,the C-mode image diameter will be about 2.3 mm, at 5 mm distance theimage diameter will be 4.6 mm, and at 7 mm distance the image diameterwill be 6.9 mm.

The host processing system, in addition to the control of the transducerelement selection and stepping around the array, will control thetransmit pulse timing, the use of any matched filter to perform echopulse compression, and the echo band pass filter processing path in thesystem. The amplified and processed analog echo information is digitizedwith enough bits to preserve the dynamic range of the echo signals, andpassed to the beamformer processing section. The beam former sectionuses stored echo data from the sparse array sampling (or alternativelythe whole complete array echo data set of 64×32 of transmit-receiveelement pairs) that exist in an aperture of interest. As the elementecho sampling continues sequentially around the circular array 1108 asshown in FIGS. 10 and 11, a number of “full trips” around the array willhave been made to collect a sufficient number of echo cross-products (upto 105 in the preferred sparse sampling method) to allow thereconstruction of one image vector line 1102. As cross-product samplingcontinues around the array, the “older” echo cross-product collectionsare replaced with new samples and the next image vector is formed. Thisprocess repeats through an angular rotation to create new image vectorswhile sampling their element cross-product contributors around thearray.

In FIG. 11, view “A” 1004 of FIG. 10 is shown which is a superpositionof the distal catheter array and the forward looking image. Transducerelements #1 and #2 shown as item 1102 show the start location for thetransmit (Tx) transducer elements. Transducer elements #12 and #13 shownas item 1106 is the start location for the Rx transducer elements. Tocollect the echo data for vector 1002, a total of 105 cross-productswill be collected from all around the array. Rotation arrow 1108 showsthe direction of Rx element stepping around the array. The Rx steppingpreferably stops at about element #52 (e.g., 64−12=52). The steppingcontinues by stepping the Rx back around after the Tx has beenincremented in the same “rotate” direction. Obviously, not allcross-product Tx-Rx terms are collected. Preferably, one takes theprimary spatial frequencies, and continues the collection to limit thecross-products to 105.

In the same manner as described in the processing of the “side looking”image, the vector echo data is processed through envelope detection ofthe echo data and rejection of the RF carrier. Finally a process ofcoordinate conversion is done to map the radial vector lines of echodata to raster scan data for video display.

This processing system, through the host control, may also accomplish“forward looking” target (such as blood cells) velocity detection byeither correlation-tracking the targets along the “forward looking”direction (with processing as earlier discussed with the “side looking”approach), or by standard Doppler processing of echo frequency shiftsthat correspond to target movement in directions parallel with the“forward looking” echo paths. The target (e.g. blood) velocityinformation may be displayed as a color on the video display; thisvelocity color information is superimposed on the image display to allowthe user to see simultaneous anatomical information and target movementinformation.

Forward Looking Sagittal-Sectional B-Mode Operation

The advantage of the “forward looking” operation of the catheter imagingdevice is in its ability to see an image of objects in front of thecatheter where possibly the catheter could not otherwise physicallytraverse. “Forward” B-mode plane imaging produces a cross-sectionalplanar “sector” view (see FIG. 3) that can exist in any plane parallelto the catheter central axis and distal to the end of the catheterarray. This imaging mode may be used, in addition, to produce image“sector” views that are tilted slightly out of plane (see FIG. 3), andas well, may produce individual or sets of image “sectors” rotatedgenerally about the catheter axis to allow the user to see a multitudeof forward image slices in a format that shows clearly themultidimensional aspects of the forward target region of interest. Thisforward B-mode imaging (as with C-mode plane imaging) utilizes the idealacoustic beam positioning for the detection and color image display ofDoppler echo signals from targets moving generally in parallel with thelong axis of the catheter device.

1. Transducer Operation

The transducer operation in creating the “forward looking” B-mode imageformat is virtually the same as discussed earlier for creating the“forward looking” C-mode image. The transducer in this “secondary” modeoperates in the length extensional (LE) resonance, utilizing the k₃₁electro-mechanical coupling coefficient to describe the couplingefficiency. As with the C-mode image creation, the number of elementsused at any time to form a wide beam pointing in the “forward” directionare selected to produce a required 60 degree FWHM beam widthperformance; the modal isolation techniques mentioned earlier againstthe higher frequency TE resonances are valid as well for this forwardB-mode imaging method.

However, where it is merely preferred to operate the “forward” C-modeimaging with high bandwidth echo signals (low bandwidth echo signals canalso be used, but with some minor loss in image resolution), it is arequirement in the “forward” B-mode imaging that only high bandwidthecho signals (echo fractional bandwidth greater than 30%) be used topreserve the “axial” resolution in the “forward” B-mode image. Thelateral resolution in the “forward” B-mode image is determined (as theC-mode image plane resolution) by the aperture (diameter of the array)used for the image reconstruction. The lateral resolution performancewill be as stated earlier (i.e. from the description of the C-modeimaging case) for various depths from the catheter distal end.

2. System Operation

The system operation in creating the “forward looking” B-mode imageformat is largely the same as discussed earlier for creating the“forward looking” C-mode image, with the difference being in the use ofthe echo signals collected in the beamforming process to create, ratherthan a C-mode image plane, a “forward” sagittal B-mode image in a planethat effectively cuts through the center of the circular array at thedistal end of the catheter.

The host processing system as shown in FIG. 9, will control the arrayelement selection and stepping process whereby one element, a twoelement pair, or other multiple elements in combination, will transmitand the same or other elements will receive the return echo information.The intended array operational mode is the LE resonant mode to send andreceive echo information in a forward direction from the end of thecatheter array. As stated earlier, the LE mode echoes produced may beisolated from the TE mode echoes through primarily frequency bandlimitations (both by transducer structural design and by electrical bandselection filters), and through the beamforming reconstruction processitself as a kind of echo selection filter.

To produce an image of the best possible in-plane resolution whileoperating in the “forward looking” sagittal B-mode, the entire arraydiameter will be used as the maximum aperture dimension. This meansthat, in general, element echo sampling will take place at elementlocations throughout the whole array in preferably a sparse samplingmode of operation to gather the necessary minimum number ofcross-product echoes needed to create image resolution of high qualityeverywhere in the reconstructed plane. By using transmit-receive echocontributions collected from elements throughout the whole catheterarray, using either a “complete data set” (e.g. 64×32), or a sparsesampling (e.g. less than 64×32) of elements, the FWHM main beam lateralresolution in the B-mode plane will be close to the 20 MHz resolution ofthe “side looking” cross-sectional image. Similarly, as stated earlierfor the C-mode image case, in the creation of the B-mode image using a10 MHz forward looking design, the FW main lobe lateral resolution inthe image plane reconstructed at a depth of 3 mm will be approximately0.39 mm, and 0.65 mm resolution at 5 mm distance.

Due to the limitation of beam diffraction available in the design using10 MHz as the echo frequency for “forward looking”, the B-mode sectorimage width that can be reconstructed and displayed with a high level ofresolution from echo contributions throughout the whole array will berelated to the distance between the reconstructed B-mode target depth inthe image sector and the distal end of the catheter. At 3 mm from theend of the catheter, the B-mode image sector width will be about 2.3 mm,at 5 mm distance the image sector width will be 4.6 mm, and at 7 mmdistance the image sector width will be 6.9 mm.

The host processing system, in addition to the control of the transducerelement selection and stepping around the array, will control thetransmit pulse timing, the use of any matched filter to perform echopulse compression, and the echo band pass filter processing path in thesystem. The amplified and processed analog echo information is digitizedwith enough bits to preserve the dynamic range of the echo signals, andpassed to the beamformer processing section. The beam former sectionuses stored echo data from the sparse array sampling (or alternativelythe whole complete array echo data-set of 64×32 of transmit-receiveelement pairs) that exist in an aperture of interest. As the elementecho sampling continues sequentially around the circular array, a numberof “full trips” around the array will have been made to collect asufficient number of echo cross-products (up to 105 in the preferredsparse sampling method) to allow the reconstruction of one image vectorline. As cross-product sampling continues around the array, the “older”echo cross-product collections are replaced with new samples and thenext image vector is formed. This process repeats through an angularrotation in the array to create new image vectors while sampling theirelement cross-product contributors around the array.

The method used for the creation of a single “forward looking” sagittalB-mode image plane may be expanded to create multiple rotated sagittalplanes around an axis either congruent with the catheter central axis,or itself slightly tilted off the catheter central axis. If enoughrotated planes are collected, the beamforming system could then possessa capability to construct and display arbitrary oblique “slices” throughthis multidimensional volume, with B-mode or C-mode visualization ineither a 2-D sector format, a 2-D circular format, or, othermultidimensional formats. The echo data volume may also be off-loaded toa conventional 3-D graphics engine that could create the desired imageformat and feature rendering that would enable improved visualization.In the same manner as described in the processing of the “forwardlooking” C-mode image, the vector echo data is processed throughenvelope detection of the echo data and rejection of the RF carrier.Finally a process of coordinate conversion is done to map the radialvector lines of echo data to a video sector-format display of the“forward looking” B-mode image.

This processing system, through the host control, may also accomplish“forward looking” target (such as blood cells) velocity detection byeither correlation-tracking the targets along the “forward looking”direction (with processing as earlier discussed with the “side looking”approach), or by standard Doppler processing of echo frequency shiftsthat correspond to target movement in directions parallel with the“forward looking” echo paths in the “forward looking” B-mode plane. Thetarget (e.g. blood) velocity information may be displayed as a color onthe video display; this velocity color information is superimposed onthe image display to allow the user to see simultaneous anatomicalinformation and target movement information.

The invention has a number of important features and advantages. Itprovides an ultrasonic imaging transducer and method that can be usedfor imaging tissue in multiple planes without any moving parts. It canoperate in both forward and side imaging modes, and it permits imagingto be done while procedures are being carried out. Thus, for example, itcan operate in a forward looking C-mode, while at the same time atherapeutic device such as a laser fiber-bundle can be used to treattissue (e.g. an uncrossable arterial occlusion) ahead of the cathetertip either by tissue ablation, or, tissue photochemotherapy. The laserpulses may be timed with the ultrasound transmit-receive process so thatthe high frequency laser induced tissue reverberations can be seen inthe ultrasound image plane simultaneously. In this way the invention candynamically guide the operator's vision during a microsurgicalprocedure.

The present invention can also be used in a biopsy or atherectomyprocedure to allow the operator to perform a tissue identification priorto tissue excision; the advantage being that the catheter or biopsyprobe device can be literally pointing in the general direction of thetarget tissue and thus aid significantly in the stereotaxic orientationnecessary to excise the proper tissue sample. The invention can also beused for the proper positioning of a radiotherapy core wire in thetreatment of target tissue that exists well beyond the distal extent ofthe catheter.

It is apparent from the foregoing that a new and improved ultrasonicimaging device and method have been provided. While only certainpresently preferred embodiments have been described in detail, as willbe apparent to those familiar with the art, certain changes andmodifications can be made without departing from the scope of theinvention as defined by the following claims.

1. A method for performing a therapeutic intravascular operationcomprising: providing a flexible elongate member comprising anultrasound imaging device of operation, wherein the ultrasound imagingdevice includes an ultrasound transducer array comprising a plurality ofultrasound transducer elements, wherein each element has first andsecond portions separated by an acoustical discontinuity, and eachelement is configured to operate in both a forward-looking mode ofoperation or a side-looking mode of operation; inserting the ultrasoundimaging device into a region of a vasculature; transmitting acousticsignals from the ultrasound imaging device in the forward-looking modeof operation; receiving acoustic echo image signals, by the ultrasoundimaging device, in the forward-looking mode of operation; and deliveringtherapy to the region of the vasculature using a therapeutic device. 2.The method of claim 1 wherein the therapeutic device deliverstherapeutic energy.
 3. The method of claim 2 wherein the therapeuticenergy comprises electromagnetic energy.
 4. The method of claim 3wherein delivering therapeutic electromagnetic energy further comprisesdelivering therapeutic electromagnetic energy through a the therapeuticdevice, wherein the device is disposed proximate to the ultrasoundimaging device.
 5. The method of claim 4 further comprising disposingthe therapeutic device proximate the ultrasound imaging device via alumen within the flexible elongate member.
 6. The method of claim 4wherein the therapeutic device comprises a laser fiber.
 7. The method ofclaim 4 wherein delivering therapeutic electromagnetic energy issynchronized with the transmitting and receiving of acoustic signals. 8.The method of claim 4 wherein delivering therapeutic electromagneticenergy comprises ablating tissue.
 9. The method of claim 4 whereindelivering therapeutic electromagnetic energy comprises performingphoto-chemotherapy.
 10. The method of claim 4 wherein the therapeuticdevice comprises a radiotherapy core wire.
 11. A method for performing aprocedure comprising the steps of: providing an ultrasound imagingcatheter comprising: a flexible elongate member including a distal endportion and a longitudinally extending axis, and a set of ultrasoundtransducer elements arranged at the distal end portion for transmittingand receiving ultrasonic energy, wherein at least one of the set ofultrasound transducer elements contains an acoustic discontinuity;providing a therapeutic device capable of delivering energy, thetherapeutic device configured for placement in proximity to theultrasonic imaging catheter; exciting ones of the set of ultrasoundtransducer elements to transmit ultrasonic acoustic waves, wherein atleast a portion of the ultrasonic acoustic waves are transmittedsubstantially perpendicular to the longitudinally extending axis;receiving echoes of the transmitted ultrasonic acoustic waves;generating, by ones of the set of transducer elements, electricalsignals in accordance with the received echoes of the ultrasonicacoustic waves; creating an image of at least a first portion of anarterial occlusion from the electrical signals; and delivering energyfrom the therapeutic device to the first portion of the arterialocclusion.
 12. The method of claim 11 wherein the delivering energy stepcomprises delivering electromagnetic energy.
 13. The method of claim 12wherein energy is delivered with sufficient intensity to ablate tissueduring the delivering electromagnetic energy step.
 14. The method ofclaim 11 further comprising disposing the therapeutic device inproximity to the ultrasonic imaging catheter via a lumen within theultrasonic imaging catheter.
 15. The method of claim 11 wherein thetherapeutic device comprises a laser fiber.
 16. The method of claim 15wherein the therapeutic device comprises a laser fiber bundle.
 17. Themethod of claim 11 wherein the therapeutic device comprises aradiotherapy wire.
 18. The method of claim 11 wherein the deliveringenergy step is synchronized in relation to the exciting and receivingsteps.
 19. The method of claim 11 wherein the acoustic discontinuitycomprises a notch.